The invention described herein relates to spent fuel storage racks and more particularly to an improved design of racks particularly adapted for storage of fuel assemblies of the type used in boiling water reactors.
The delays in undertaking the reprocessing of reactor spend fuel in the United States has required utilities to better utilize the spend fuel storage space at a reactor site in a way to permit the storage of larger quantities of fuel in the same given area. The delays also have provided the economic incentive to increase the storage capacity and thus better control the handling and disposition of spent fuel and costs associated therewith. Initially, plant designers typically included at the reactor site, a spent fuel pool sized to receive a number of spent fuel assemblies less than the total amount expected to be removed from the reactor during its lifetime. The fuel assemblies were located on centers or at a pitch such that the space between assemblies together with the water surrounding each fuel assembly was sufficient to maintain the fuel in a non-critical condition. At this spacing, subcriticality was maintained by utilizing only water as a moderator. As the need for compact storage increased, the first stage of capacity expansion included the use of stainless steel cells for containing each fuel assembly thus permitting reduced spacing between fuel assemblies. This reduction increased the storage capacity by simply changing the design of storage racks without increasing the size of the storage pool. As decisions concerning reprocessing continued to be delayed, greater compaction of fuel assemblies into the allotted pool space was accomplished by applying neutron absorbing materials to the walls of the stainless steel containers or cells which were made to a size to just accept a fuel assembly. This design permitted cells to be spaced on a pitch even less than previous rack designs thus increasing the storage capacity to the extent where the storage pool could accommodate about 10 years of spent fuel.
To provide stability and support to prior spent fuel racks, a common arrangement was such that the spent fuel cells were laterally spaced from each other by structural members extending in X and Y directions to thus provide cell support. The egg crate arrangement of cells thus formed allows one fuel assembly to be located in each cell designed to specific tolerances. However, the structural members still utilize space which otherwise could be used more efficiently for fuel assembly storage purposes. Also, fuel racks of the foregoing design contain substantial labor and material content which is reflected in greater manufacturing costs.